AND DISPERSION IN ICELAND SPAR. 
449 
Thus near minimum deviation the change produced in /x amounts to about '0003 
while at grazing incidence it is only about '0001. 
Of course an error of the same kind occurs in the values of p, 3 . They, however, 
were determined from observations at nearly grazing incidence. 
They may then be slightly too great. 
To correct them completely for this error we should have to reduce the theoretical 
values by a small quantity nearly the same for all; while the experimental values 
require reducing by quantities which are greatest near minimum deviation, and decrease 
as we approach grazing incidence until they reach about the values of the corrections 
applied to /x x and /x 3 . 
The greatest error for the ray g does not exceed '0002, so that the results of theory 
and experiment for g would be brought into very close agreement by supposing the 
violet rays of the light emerging from the collimator to be inclined to the red at angles 
not greater than 45". 
Thus, allowing for this probable divergency of the green and violet rays, it appears 
that Huyghen’s construction represents the result of experiment for the three rays of 
the hydrogen spectrum to a degree of approximation comparable with the probable 
error of the experiments.* 
* In the abstract printed in the ‘ Proceedings of the Royal Society,’ I had. assumed that the violet rays 
issuing from an achromatic lens, focused so as to make the orange rays parallel, were convergent. Prom 
this it followed that the correction for want of parallelism tended to increase the difference between theory 
and experiment, and led me to the inference that Huyghen’s construction might be true for the red rays 
and yet differ appreciably from the truth for light of shorter wave length. Professor Stokes has since 
pointed out to me that the violet rays are in reality divergent, and that the error introduced by assuming 
them to be parallel really tends to correct the differences observed between theory and experiment, and 
so leads to the inference in the text that Huyghen’s construction is true for the three hydrogen rays 
within the limits of experimental error. 
